Frequently used for fill level measurement, especially for limit level registration, are vibronic measuring devices having an oscillatable unit excitable to execute mechanical oscillations. The oscillatable unit is, in the case of applications in liquids, as a rule, embodied as a so called oscillatory fork having two rods secured on a membrane and acting as the tines of the fork, while for bulk goods both oscillatory forks as well as also single rods having only one resonating rod are applied. Serving for oscillation excitement are, most often, piezoelectric or magnetoelectric drives, which excite the oscillatable unit to its resonant frequency. The piezoelectric drive in the case of oscillatory forks is placed on the rear side of the membrane. The membrane is, in such case, rather thin, in order that a good oscillatory energy transmission takes place between the resonator and the drive. Usually, the membrane is, for instance, 1 to 2 mm thick and its diameter corresponds to 0.5 inch to 1.5 inch. The strength of such membranes is limited in the case of high process pressures and lies typically under 10 MPa.
In the German patent, DE 3878473 T3, a drive technique is described, wherein the oscillatory fork is driven with two piezoelectric elements arranged directly in the tines. An element applied as drive and a piezoelectric element applied as receiving element are respectively mounted on walls of hollow spaces in the tines. In this way, the forces, which the piezo drive delivers, are not transmitted via a membrane, but, instead, directly exerted in root regions of the tines. In this case, the membrane plays a lesser role in the oscillatory system of the sensor and can be as thick as desired. This helps clearly to improve the strength of the sensor under pressure. The application of the piezoelectric drive, and receiving, elements is, in such case, however, relatively difficult to carry out, since the piezoelectric elements must be adhered to the wall and positioned exactly. The exact positioning of the piezoelements plays an important role, since the piezo drive must produce a bending deformation in the transitional region between tines and membrane. Additionally, such a drive is only efficient in the case of metal oscillatory forks.